Newt Gingrich is expected to withdraw next week from the US Republican presidential contest, two advisers to the former speaker of the House of Representatives have said.

The decision by Gingrich emerged on Wednesday, a day after Mitt Romney added to his commanding lead in the Republican nomination race with primary wins in five states.

Gingrich's advisers spoke to The Associated Press news agency on the condition of anonymity given the sensitivity of the situation. The advisers said he will formally end his campaign next Tuesday and will likely endorse Romney.

The former House speaker did not formally withdraw from the race but said on Wednesday he is now campaigning as a "citizen". He did not explain what he meant.

Gingrich had campaigned heavily in Delaware, one of five states to hold presidential primaries on Tuesday, but lost by nearly 30 percentage points.

"I don't think we can lose by 30 points in Delaware and feel good about it," Gingrich adviser Bob Walker, a former US congressman, said on Tuesday before the polls closed.

Despite a string of losses since Gingrich's upset victory in the South Carolina primary in January, he had vowed to stay in the race until his party's nominating convention in late August.

His withdrawal clears the way for Romney to claim the unofficial mantle of the Republican nominee in November's election, putting an end to a bitter primary campaign.

Gingrich, who rose to be the most powerful Republican in the US during his leadership of the House in the 1990s, finished first in just two of the 36 states that have voted in the 2012 presidential contests.

'Never down'

Throughout the campaign, Gingrich's spirits seemed to float above his sinking fate. At a debate in February, Gingrich, known for his caustic takedowns of opponents and media alike, chose one word to describe himself: "Cheerful".

"I never got the sense that he was quote-unquote down," said adviser Charlie Gerow. "I got the sense on a couple of occasions that he was tired. Really tired."

His candidacy never lacked drama. Days before Gingrich's shocking victory in South Carolina, his second wife appeared in a television interview, saying that Gingrich desired an open marriage.

Gingrich vociferously denied the charge at a debate, the forum that helped give lift to his campaign.

Last summer, Gingrich watched a staff exodus as advisers carped about his decision to take a Greek cruise with his wife, Callista, losing time that could be spent raising money and meeting voters.

Gingrich said the trip showed that he was running an unconventional campaign. By stumping for a moon colony, encouraging poor students take up janitorial work, and preparing for at least one debate by watching the film "Bridesmaids," Gingrich confirmed his own assessment.

Now viewed as a sideshow, Gingrich's campaign was once at center ring.

In December, polls showed Gingrich soaring ahead of Romney, but after a barrage of ads from the Restore Our Future "Super PAC" that cast Gingrich as a Washington insider with questionable ethics, he plummeted to finish fourth in the Iowa caucuses on January 3.

More than any other presidential candidate, Gingrich has felt the impact of independent Super PACs, the political action committees that have no limits on how much money they can raise or spend in support of candidates.

Throughout the primary season, Gingrich has depended on the largesse of a PAC called Winning Our Future, which received at least $21.5m in donations from billionaire casino mogul Sheldon Adelson and his family.

At the same time, a pro-Romney PAC, Restore Our Future, spent $19m on ads attacking Gingrich.